In-situ randomization and recording of wafer processing order at process tools

ABSTRACT

Wafer order is randomized in-situ by use of a separate wafer staging area and randomly shuffling wafers to and from this staging area to shuffle the processing order of the wafer lot. Positional data is captured for each wafer at both the send and receive ends of the process.

BACKGROUND AND SUMMARY OF THE INVENTION

[0001] The present invention relates to integrated circuit structuresand fabrication methods, and more particularly to wafer level yieldanalysis.

[0002] The economics of modern integrated circuit microfabrication areunusual. Every integrated circuit is tested at the end of manufacturingprocess, and a significant fraction of the integrated circuits must bediscarded. (Typical yield loss may be a few percent to several tens ofpercent). Since the maximum number of wafers which can be processed perhour by the very expensive capital equipment is nearly fixed, the yieldof good devices per wafer is a key determinant of profitability. Thismeans that any improvement in yield is of great interest.

[0003] One of the key elements in yield optimization is fault analysis.Since there are numerous processing steps and machines which can causeyield loss, it is not necessarily obvious which processes are performingoptimally and which are not. Test structures can indicate physicalconditions, such as shorts, opens, degraded sheet resistance, ordegraded contact resistance, but it still may not be apparent what, inthe processing conditions, is leading to these physical conditions inthe resulting structure. It is also very difficult to detect cooperativeeffects, where two processes are both out of tune in a way whichcombines with disastrous synergy.

[0004] An important recent innovation in fault analysis has been waferrandomization. Wafers are usually routed through a fab in lots of e.g.25, and some fault analysis information can be derived from studyingvariation over time, or from lot-to-lot comparisons; but when the orderof wafers within a lot remained fixed (as was formerly traditional), theinformation which could be derived from intralot comparison was minimal.For example, it was easy to observe (for example) that failure ratesincreased in the last half of a lot, but difficult to infer whichmachine might have been the source of this trend. However, by shufflingthe order of wafers before critical processing steps, vastly moreinformation could be derived from fault correlation analysis.(Mathematically, n wafer randomizations in a lot of k wafers allowsmultidimensional correlation and filtering operations, using ann+1-dimensional sequence vector.)

[0005] In order to take advantage of the fault correlation potential ofwafer randomization, the wafer sequence data for each processing stepmust be retained. The simplest way to do this is by automaticallyvalidating the identity of every wafer before critical processing steps,and recording the resulting data. This provides the data for thesequence vector, and has the side benefit of preventing wafer mix-upsand potential misprocess scrap of mixed lots.

[0006] Also, since the spatial position of each wafer is consistentlyfixed at critical steps (since the machines normally reference the wafernotches or flats), fault analysis can (and should) also correlate trendsto spatial locations on the wafer.

[0007] The input to this improved correlation capability is not limitedto failures. Parametric data (e.g. from probe or device performancetesting) can also be correlated back into the different stages in theprocess, to permit further optimization of yield (and optionally alsoperformance).

[0008] A benefit of this is improved engineering productivity: processengineers can focus on leads from correlation analysis, instead ofhaving to guess which step or combination of steps may be out of tune.The result is quicker and more efficient detection of lot yieldproblems. (A different way to think of this is that every production lotcan also be analyzed as an experimental lot, WITHOUT any loss of yieldor of production throughput.)

[0009] The original form Wafer Sleuth technology from Texas Instruments,used in combination with randomizer stations for wafer shuffling,provides many of these benefits. The present application implementsfurther improvements in fault correlation, and in associated productionsteps.

In-situ Randomization and Recording Of Wafer Processing Order at ProcessTool

[0010] The present application discloses an innovative way to randomizewafers in-situ during processing. The preferred embodiment is used withtools that have wafer pick and place capability, meaning they can selectany wafer from a cassette rather than just accessing them in aparticular order. Randomization of wafer order is provided by randomlypulling wafers from any slot on the send (incoming) cassette, orreplacing wafers to a random slot in the receive (outgoing) cassette.Further randomization can be obtained by combining these methods.

[0011] In another embodiment, which uses a wafer tool with pick andplace capability, wafers are completely randomized at the load and/orunlead stations prior to, or after, processing. This method requiresthere be at least one empty slot for a wafer, or another wafer stagingarea available. This method is best used in a process where the timedelay does not affect throughput, such as when the randomization is donewhile other wafers are occupying the needed tool.

[0012] In yet another embodiment, which applies to tools without pickand place capability, and for lot sizes of less than 25 (or the maximumnumber of wafer slots in the cassette holder, meaning free slots existin the holder), wafers use a second temporary holding area, and wafersare pulled directly from the send cassette and either randomly stored orsent to processing. Wafers can then be pulled for processing randomlyeither from the next slot in the send cassette, or form the stagingarea. Wafers can also, at the receive end after processing, be randomlyplaced in the cassette or in a staging area, and wafers thereafter canbe returned to the receive cassette either from the process tool or fromthe holding area.

[0013] In order to keep track of wafer processing order, the wafernumber, or wafer ID, is read inside each processing tool. In the case ofa multiple process chamber tool, the wafer number or ID is read at eachprocess chamber. Note that both wafer ID and wafer order are tracked inthe preferred embodiments. The resulting information about the sequenceof wafers through each critical processing step can then be used forfault correlation.

[0014] Advantages of the disclosed methods and structures, in variousembodiments, can include one or more of the following:

[0015] prevents wafer mix-up from ex-situ shuffling;

[0016] reduces manufacturing time by eliminating separate randomizationstep;

[0017] saves fab floor space used for randomizers.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The disclosed inventions will be described with reference to theaccompanying drawings, which show important sample embodiments of theinvention and which are incorporated in the specification hereof byreference, wherein:

[0019]FIG. 1 shows a flow chart for key steps in the innovative waferfabrication process.

[0020]FIG. 2 shows a flow chart for key steps in the innovative waferfabrication process.

[0021]FIG. 3 shows a flow chart for key steps in the innovative waferfabrication process.

[0022]FIG. 4 shows a flow chart for key steps in the innovative waferfabrication process.

[0023]FIG. 5 shows a wafer process that incorporates the preferredembodiment.

[0024]FIG. 6 shows an innovative process tool that randomizes thewafers.

[0025]FIG. 7 shows a flowchart for another embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] The numerous innovative teachings of the present application willbe described with particular reference to the presently preferredembodiment. However, it should be understood that this class ofembodiments provides only a few examples of the many advantageous usesof the innovative teachings herein. In general, statements made in thespecification of the present application do not necessarily delimit anyof the various claimed inventions. Moreover, some statements may applyto some inventive features but not to others.

[0027] The innovative concepts are described with reference to thepreferred embodiment and the figures.

[0028] Advanced semiconductor yield analysis requires that the waferprocessing order be known at every machine or processing chamber. Thus,if a machine has multiple processing chambers, the wafer processingsequence must be known at each chamber. In order for yield analysis todistinguish between different tools, the wafer processing order israndomized. This makes it easier to trace systematic defects in wafersback to the tools which caused them.

[0029] In the preferred embodiment, the wafers are randomized in-situ.This is done by modifying the programming of the loading function forthe cassettes that hold the wafers. Tools with pick and place capability(i.e., access to any slot in the cassette at any timne, rahter thanaccessing the slots sequentially only) can have their readers upgradedand/or software upgraded to perform the randomization and wafer trackingthat is necesary for practicing the invention, including randomizationand wafer tracking tasks.

[0030] There is typically a send cassette and a receive cassette. Thesend cassette contains incoming wafers going to the tool, and thereceive cassette contains outgoing wafers coming from the tool. Thewafer numbers are read inside the actual process tool. In the case ofmultiple chambers inside a tool, the wafer numbers are read in eachchamber. This allows the sequence of wafers to be known at the tool orthe individual chambers, as needed.

[0031] Except where noted, the following discussion refers to singleprocessing chamber tools. However, it should be understood that the sameconcepts can be applied to multiple chamber tools, as discussed furtherbelow.

[0032] In the preferred embodiment, wafers are randomized in-situ. Awafer tool with pick and place capability is used. Wafer randomizationis provided at either the send cassette, the receeive cassette, or both.Wafers are randomly pulled from any slot on the send cassette and senton for processing. This privides one level of randomization for thewafer order. Wafers may also be randomly placed in any open slot in thereceive cassette after processing, which provides another layer ofrandomization. Wafers may also be randomized at both the send andreceive cassettes, furhter randomizing the order. Positional data andwafer ID are captured for both the send and receive cassettes.

[0033]FIG. 1 shows a process flow for the innovative randomizationtechnique. The wafers arrive at the process tool, their order in thecassette known (step 102). A wafer is randomly pulled from a slot in thesend cassette (step 104). The wafer's ID is recorded (step 106) and thewafer is processed in the tool (step 108). The wafer is then randomlypoaced in an available slot in the receive cassette (step 110), and thewafer's position in the cassette is recorded (step 112). This exampleflow uses randomization at both ends of the process (send and receive).It should be noted that either randomization alone will providesufficient randomization, and both randomization techniques need not beused simultaneously.

[0034] In another embodiment, the wafer order is completely randomizedat the load and/or the unload stations prior to (or after, in the unloadstation's case) processing. This requires at least one empty slot in thecassette, or another wafer staging area to be available (such as atemporary holding cassette, for example). Again, the wafer ID andpositional data are captured at at least one process point that allowsthe wafers to be tracked by software both into and out of the tool. Thisoption is preferable in a process where a delay for randomization willnot affect throughput or cycle time, such as when another lot isundergoing processing.

[0035]FIG. 2 shows a flowchart with key steps to the innovative process.the wafers arrive at the processing tool in the send cassette (step202), their order known. The wafers are then entirely randomized byshuffling them within the send cassette using a tool with pick and placecapability (step 204). The wafer order within the send cassette is thencaptured (step 206). It should be noted that the wafer ID may becaptured within the process tool itself, depending on the implementationused. The wafer undergoes processing (step 208). Once all wafers areprocessed, the wafer order within the receive cassette is againrandomized by teh pick and place tool (step 210), and the wafer order isagain captured (step 212).

[0036] It should be noted that the above two embodiments mentioned maybe combined to provide even more thoroughly randomized wafer lots,though the cost in time and implementation may make such a combinationundesireable.

[0037] For a tool without pick and place capability (i.e., for toolsthat can only access wafers by retracting or replacing wafers belowactual product wafers, not above them), and on lot sizes of 25 wafers orless (i.e., a small enough lot to fit into a single wafer cassette), asecond temporary holding cassette is used to randomize wafer order oncethe send cassette is inside the tool. (Note that any separate waferstaging area could also be used.) Wafers pulled directly from the sendcassette can either be randomly stored in the holding cassette or senton for processing. The random selection of whether to shelve a wafer orsend it directly to processing shuffles the wafers, allowing waferanalysis to distinguish different tools. The processing order of thewafers is recorded in the processing tool (or at each individual chamberin multichamber tools).

[0038] Instead of pulling wafers only from the send cassette, once thereare wafers in the holding cassette the loading tool can randomly pullwafers either from the send cassette or the holding cassette. Thisfurther randomizes wafer order.

[0039] After processing, the wafers may be randomly returned to eitherthe receive cassette or a second hold cassette (or other wafer stagingarea). Thus, after processing, a given wafer may either go directly intothe receive cassette, or it might go to the hold cassette. Once thereare wafers in the hold cassette, wafers may be randomly returned to thereceive cassette either directly from processing, or from the holdingcassette. (In the preferred embodiment, all wafers must end up in thereceive cassette.) This adds another shuffling step to the wafer order.The positional data for the wafers is recorded so that the wafer orderis known once all wafers are in the receive cassette. (In the preferredembodiment, the positional data is captured for both the send and/or thereceive wafer order.)

[0040]FIG. 3 shows a flow chart of an embodiment of the presentinnovations. When a wafer send cassette arrives at a processing tool(step 302), the top wafer of the lot (for a tool without pick and placecapability) is removed and randomly sent either to processing or to atemporary wafer staging area (step 304). Once there are wafers in thestaging area, wafers may be pulled randomly from either the sendcassette or from the staging area and sent to processing (step 306).This choice adds further randomization to the wafer order. If there areno wafers in the staging area, then the next wafer from the sendcassette is randomly sent either to processing or to the wafer stagingarea (step 308). The process is continued until all wafers have beensent to processing. Note that the wafer order is captured as they enterthe processing tool or chamber, depending on the configuration of thetool.

[0041]FIG. 4 shows a flow chart for when the wafers complete processingin the tool's process chamber. Processed wafers are returned at randomto either the receive cassette or to a separate wafer staging area (step402). Once there are wafers in the wafer staging area, then wafers aresent to the receive cassette, at random, from either the process chamberor from the wafer staging area (step 404). This adds furtherrandomization to the wafer order. If there are no wafers in the waferstaging area, then wafers are pulled from processing and sent at randomto either the receive cassette or to the wafer staging area (step 406).Note that this process continues until all wafers are in the receivecassette. The wafer order is captured (by recording wafer IDs) for thefinal wafer position in the receive cassette. wafer ID capture can beaccomplished, for instance, by scanners and bar codes on the wafers, orother computer readable means. In the preferred embodiment, the wafer IDsi captured thorugh hardware modifications by installing OCR (opticalcharacter reader), laser, or CCD based barcode scanners or CCD based 2Dreaders. The readers are strategically located in the tool where thereis only one wafer ID capture needed per wafer, and the rest of thetracking is done through software. The positional and process order datarequires software upgrades to incorporate it into a process, where thehost system for the wafer fab tool ieeps track of the wafer history andstores informatino locally or sends informatin over the LAN through acommunication protocol.

[0042]FIG. 5 shows a wafer process that incorporates the preferredembodiment. Wafers 502 are introduced to front end processing 504, whichincludes at least one, usually several, front end process steps. In thepreferred embodiment the wafer order is randomly shuffled at criticalprocess steps where defect generation is possible or likely. The waferorder is recorded and tracked throughout these steps, generating a wafersequencing vector 506 that contains the wafer order information at anygiven point in the process. After processing, the wafers go throughtesting 508, which generates fault and parametric data 510. Next, theinnovative process generates a correlation 512 of the sequence data withthe fault data to generate process feedback data 514. This data is usedby engineering to generate modifications 516 to the process aimed atreducing the defects detected in the wafers.

[0043]FIG. 6 shows an innovative process tool that randomizes thewafers. A wafer send cassette 602 has slots for individual wafers, andan arm 604 that moves the wafers. The tool may be capable ofpick-and-placing the wafers anywhere, or in other embodiments can onlyaccess wafers in a particular order. A wafer staging area 606 may beused for shuffling the wafers. In this examnple, incoming wafers areshuffled before going to the processing area 608, or outgoing wafers areshuffled as they are placed in (in this case) the receive cassette 602.

[0044]FIG. 7 shows a flowchart for another embodiment. For amulti-chamber tool that uses the same process for each chamber, waferscan be randomly processed in any of the chambers. the wafers enter themulti-chamber tool (step 702), and are randomly rocessed in any of thechambers (step 704). Positional dat ais captured for the wafer chamberprocess order (step 706).

[0045] Another embodiment applies to processing tools with multipleprocessing chambers that use the same process for each of its chambers.For such a tool, each of the wafers can be randomly processed in any ofthe chambers. Of course, the positional data must be captured for thewafer chamber process order.

[0046] Automatic wafer randomization coupled with wafer number readingat the process chamber or tool helps prevent uncertainty due to wafermix-up between the reading of wafer order (which typically occursex-situ in a separate apparatus) and actual processing. It also reducesmanufacturing time by eliminating the separate randomization and readstep. This also allows the randomizer apparatus to be eliminated fromthe fab floor space. One randomizer typically occupies 4 square feet ofextremely expensive fab floor space. A sorter occupies 9 square feet. Atypical fab may have 50 randomizers.

[0047] The innovative in-situ ramdomization and tracking method alsoreduces defect density by eliminating an extra wafer handling step.Implementation of these innovative concepts in some cases requires onlysoftware modification.

[0048] According to a disclosed class of innovative embodiments, thereis provided: A method of wafer processing, comprising the steps of: whena wafer lot arrives at a processing tool in a send cassette, randomizingsaid lot by transporting individual wafers thereof variously, withinsaid tool, from said cassette either to a processing position or to awafer staging area. Also provided is a processing tool which isprogrammed to perform this method.

[0049] According to another disclosed class of innovative embodiments,there is provided: A method of wafer processing, comprising the stepsof: when a wafer lot arrives at a processing tool in a send cassette,randomizing said lot by transporting individual wafers thereofvariously, within said tool, from said cassette to a processingposition, or from said cassette to a wafer staging area, if said waferstaging area is empty, or from said wafer staging area to saidprocessing position. Also provided is a processing tool which isprogrammed to perform this method.

[0050] According to another disclosed class of innovative embodiments,there is provided: A method of wafer processing, comprising the stepsof: when a wafer lot arrives at a processing tool in a send cassette,randomizing said lot by transporting individual wafers thereofvariously, within said tool, from said cassette either to a processingposition or to a wafer staging area. Also provided is a processing toolwhich is programmed to perform this method.

[0051] According to another disclosed class of innovative embodiments,there is provided: Also provided is a processing tool which isprogrammed to perform this method.

[0052] According to another disclosed class of innovative embodiments,there is provided: A method of wafer processing, comprising the stepsof: randomizing a wafer lot by transporting individual wafers thereofvariously, within a processing tool, from a processing position to awafer cassette, from a wafer staging area to a wafer cassette, or fromsaid processing position to said wafer staging area. Also provided is aprocessing tool which is programmed to perform this method.

[0053] According to another disclosed class of innovative embodiments,there is provided: A fabrication method, comprising the steps of:varying the history of wafers within a lot by transporting individualones thereof within a processing tool with multiple processing chambers,among said chambers and wafer cassette and/or staging locations invarious different sequences. Also provided is a processing tool which isprogrammed to perform this method.

Modifications and Variations

[0054] As will be recognized by those skilled in the art, the innovativeconcepts described in the present application can be modified and variedover a tremendous range of applications, and accordingly the scope ofpatented subject matter is not limited by any of the specific exemplaryteachings given, but is only defined by the issued claims.

[0055] Alternatively, it is possible to adapt the disclosed in-situshuffling methods to tools which do not capture wafer identification andsequence, but instead follow a predetermined algorithm. However, this isless desirable, since such an architecture is more vulnerable tounrecorded changes in wafer sequence due to manipulation by engineers ortechnicians.

[0056] The wafer order can be randomized at non-ctritical steps as wellas critical steps. Also, tThe wafer order, instead of being randomlyordered by the innovative tool, may instead be set to a reorder thewafers into a particular sequence. This variation is useful, forinstance, where there are particular points in the process where mererandomized wafer order may not accentuate defects, or may not indicateat exactly which process step the faults were generated. By selecting aparticular order for the wafers, or by selecting sets of particularorders for the wafers at selected process steps, fault generating stepscan more easily be identified.

[0057] The innnovatiosn of the present application are also applicablein processes where some wafer randomization is done ex-situ, some donein-situ. Wafer order data must of course be known at all phases.

[0058] Additional general background, which help to show the knowledgeof those skilled in the art regarding variations and implementations ofthe disclosed inventions, may be found in the following documents, allof which are hereby incorporated by reference:

[0059] Coburn, PLASMA ETCHING AND REACTIVE ION ETCHING (1982); HANDBOOKOF PLASMA PROCESSING TECHNOLOGY (ed. Rossnagel); PLASMA ETCHING (ed.Manos and Flamm 1989); PLASMA PROCESSING (ed. Dieleman et al. 1982);Schrnitz, CVD OF TUNGSTEN AND TUNGSTEN SILICIDES FOR VLSI/ULSIAPPLICATIONS (1992); METALLIZATION AND METAL-SEMICONDUCTOR INTERFACES(ed. Batra 1989); VLSI METALLIZATION: PHYSICS AND TECHNOLOGIES (ed.Shenai 1991); Murarka, METALLIZATION THEORY AND PRACTICE FOR VLSI ANDULSI (1993); HANDBOOK OF MULTILEVEL METALLIZATION FOR INTEGRATEDCIRCUITS (ed. Wilson et al. 1993); Rao, MULTILEVEL INTERCONNECTTECHNOLOGY (1993); CHEMICAL VAPOR DEPOSITION (ed. M. L. Hitchman 1993);and the semiannual conference proceedings of the Electrochemical Societyon plasma processing.

What is claimed is:
 1. A method of wafer processing, comprising thesteps of: when a wafer lot arrives at a processing tool in a sendcassette, randomizing said lot by transporting individual wafers thereofvariously, within said tool, from said cassette either to a processingposition or to a wafer staging area.
 2. The method of claim 1, furthercomprising the contemporaneous step of recording process sequence datafor said wafers.
 3. The method of claim 1, further comprising thesubsequent step of correlating fault and/or parametric data with processsequence data resulting from said randomization step.
 4. The method ofclaim 1, further comprising the subsequent step of correlating faultand/or parametric data with process sequence data resulting from saidrandomization step, and controlling process parameters accordingly. 5.The method of claim 1, further comprising the contemporaneous step ofrecording positional data for said wafers.
 6. A method of waferprocessing, comprising the steps of: when a wafer lot arrives at aprocessing tool in a send cassette, randomizing said lot by transportingindividual wafers thereof variously, within said tool, from saidcassette to a processing position, or from said cassette to a waferstaging area, if said wafer staging area is empty, or from said waferstaging area to said processing position.
 7. The method of claim 6,further comprising the contemporaneous step of recording processsequence data for said wafers.
 8. The method of claim 6, furthercomprising the subsequent step of correlating fault and/or parametricdata with process sequence data resulting from said randomization step.9. The method of claim 6, further comprising the subsequent step ofcorrelating fault and/or parametric data with process sequence dataresulting from said randomization step, and controlling processparameters accordingly.
 10. A method of wafer processing, comprising thesteps of: when a wafer lot arrives at a processing tool in a sendcassette, randomizing said lot by transporting individual wafers thereofvariously, within said tool, from said cassette either to a processingposition or to a wafer staging area.
 11. The method of claim 10, furthercomprising the contemporaneous step of recording process sequence datafor said wafers.
 12. The method of claim 10, further comprising thesubsequent step of correlating fault and/or parametric data with processsequence data resulting from said randomization step.
 13. The method ofclaim 10, further comprising the subsequent step of correlating faultand/or parametric data with process sequence data resulting from saidrandomization step, and controlling process parameters accordingly. 14.A method of wafer processing, comprising the steps of: randomizing awafer lot by transporting individual wafers thereof variously, within aprocessing tool, from a processing position to a wafer cassette, from awafer staging area to a wafer cassette, or from said processing positionto said wafer staging area.
 15. The method of claim 14, furthercomprising the contemporaneous step of recording process sequence datafor said wafers.
 16. The method of claim 14, further comprising thesubsequent step of correlating fault and/or parametric data with processsequence data resulting from said randomization step.
 17. The method ofclaim 14, further comprising the subsequent step of correlating faultand/or parametric data with process sequence data resulting from saidrandomization step, and controlling process parameters accordingly. 18.A fabrication method, comprising the steps of: varying the history ofwafers within a lot by transporting individual ones thereof within aprocessing tool with multiple processing chambers, among said chambersand wafer cassette and/or staging locations in various differentsequences.
 19. The method of claim 18, further comprising thecontemporaneous step of recording process sequence data for said wafers.20. The method of claim 18, further comprising the subsequent step ofcorrelating fault and/or parametric data with process sequence dataresulting from said varying step.
 21. The method of claim 18, furthercomprising the subsequent step of correlating fault and/or parametricdata with process sequence data resulting from said varying step, andcontrolling process parameters accordingly.
 22. A processing tool whichis programmed to perform the method of claim
 1. 23. A processing toolwhich is programmed to perform the method of claim
 6. 24. A processingtool which is programmed to perform the method of claim
 10. 25. Aprocessing tool which is programmed to perform the method of claim 14.26. A processing tool which is programmed to perform the method of claim18.